X-ray apparatus.



Cl E. CAMPBELL.

X-RAY APPARATUS.

APPLICATION FILED MAY 5. 19l3.

Patented Aug. 6, 1918.

4 SHEETSSHEET 1.

4 SHEETS-SHEET 2.

C. E. CAMPBELL."

X-RAY APPARATUS.

APPLICATION FILED MAY 5. I913. 1,27%,852w Patented Aug. 6, 1918.

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X-RAY APPARATUS.

APPLICATION FILED MAY 5.19:3.

P I w a m mg C. E. CAMPBELL.

X-RAY APPARATUS.

APPLICATION FILED MAY 5. 1913.

1 ,274,852. 7 Patented Aug. 6, 1918.

4 SHEETS-SHEET 4.

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cnmss E. CAMPBELL, or mum, uassaenusm'rs.

' x-mnr APPARATUS.

To all whom it may concern Be it known that 1, CHARLES E. CAMP- BELL, acitizen of the United States, residing at Lynn, in the county of Essexand State of Massachusetts, have invented certain new and usefulImprovements in X-Ray Apparatus; and I do hereby declare the followingto be a full, clear, and exact description of the invention, such aswill enable others skilled in the art to which it appertains to make anduse the same.

The present invention relates to X-ray apparatus, and more particularlyto apparatus for stepping up and rectifying an alternating current tosupply a hlgh tension unidirectional current for exciting an X- raytube.

The object of the present invention is to reorganize and improveapparatus of this type. The improvements particularly relate to thewinding of the step-up transformer and the current regulating means forthe transformer; to the rectifier for rectifying the high tensionalternating secondary current of the transformer into a high tensionunidirectional current for exciting an X-ray tube; to a voltmeter andits connections for determining the voltage of the high tension currentdelivered to an X-ray tube; to a switch in the primary circuit of thetransformer for changing the direction of flow of the uni-directionalcurrent, and a polarity indicator for indicating the proper position ofthe switch; to the adaptation of the polarity indicator for use as asynchronizer for the rectifier driving motor; to mechanism for cuttingoff the current from the X-ray tube after a predetermined exposure; tothe construction of the terminals to which an X-ray tube is connected,and to certain other features of construction hereinafter described andclaimed, the advantages of which will be apparent to those skilled inthe art.

In the drawings, which illustrate the preferred embodiment of theinvention, Figure 1 is a diagrammatic view showing the wiringconnections; Fig. 2 is a side elevation of the apparatus, one of thepanels being removed to show the motor and rectifier; Fig. 3 is a viewtaken from the inside of the casing, showing the pulleyconnections forturning the spark-gap arms; Fig. 4 is a detail view of the high tensionrectifier; Fig. 5 is a sectional view of one of the high tension currentterminals for the X-ray tube;

Specification of Letters Patent. Application filed May 5, 1913. SerialNo. 195,535.

, Patented Aug. 6, 1918.

6 is a section of the regulator terminal; F1g. 7 1s a cross-sectionalong the line 7- 7 of Flg. 5 and Fig. 8 is a detail view showmg thetlme-controlling switch.

In the illustrated embodiment of the invention, the apparatus isinclosed in a wooden casing 1. On the front of the easing is theswitchboard 2 and below it, a

transformer so as to furnish a high tension uni-directional current forexciting the X- ray tube. The X-ray tube 10 is mounted upon any suitablesupport and is connected, through flexible leads, to the anode and cathoe terminals.

The current is derived from a singlephase three-wire alternating currentsystem. The voltage between the two outside wires 11 and 12 is 220volts, and the voltage between the middle or neutral wire 13 and each ofthe outside Wires is 110 volts. A switch (not shown) external to theapparatus is inserted between the supply Wires 11, 12 and 13 and thesupply mains. The transformer 5 is designed to operate under a primaryvoltage of 220 volts, and it derives its primary current from the twooutside wires 11 and 12 of the supply circuit.

The transformer current is led through a two-pole double throwreversi-ngswitch 14, a time switch 15, the rheostat 3 and a voltageregulator 16, to the transformer primary. The high tension secondarycurrent of the transformer is rectified by the rotary rectifier into ahigh tension unidirectional current, which is delivered through theexternal anode and cathode terminals A and C to the X-ray tube. Theuni-directional current delivered to the X ray tube is a fluctuatingdirect current consisting of the higher-voltage part of each wave of thealternating current. In order to enable the operator to readilydetermine the voltage of the uni-directional current supplied to theX-ray tube, a volt-meter 20 is connected with the secondary of thetransformer 5. The time switch 15 is controlled ways by atime-controlling device 21 which may be set to cause the primary circuitof the transformer to be opened after a predetermined time.

The motor 6, which drives the rotary rectifier 7, is designed to operateupon 110 volts and is connected between one of the outside wires and theneutral wire of the three-Wire supply system, through a starting switch22. The motor, the construction of which is not a part of the presentinvention, has its armature wound like a single phase converter. Themotor has a four-poc laminated field which is so wound that the motorcan be started and brought up to speed as a commutated single phasealternating current motor, and then after the motor is brought up tospeed, it can be run as a S111- gle phase synchronous motor. When thestarting switch 22 is thrown up into contact with the switchcontacts 23and 24 the current is led to the motor commutator 25 and the startingfields, andthe motor is started as a comlnutated alternating currentmotor. When the starting switch 22 is thrown down into contact with thestationary switch contacts 26, 27, 28 and 29, the current is ledto theslip rings 30 and the running field-winding, and the motor is run as asynchronous motor in step with the exciting current of the transformer5.

When the motor comes up to synchronous speed and the switches in theprimary clrcult of the transformer are closed, it is a matter of chancewhether the anode and cathode terminals A and C are positive andnegative in sign respectively, or vice versa. Th1s is due to the factthat the angular position of the rotor of a four-pole synchronous motorof this type may, with relation to the phase of the current wave,difi'er 90 of the angular position of the rotor or 180 of the currentwave each time the motor is used, and consequently the angular positionof the disk may, with relation to the instantaneous polarity of thestationary rectifier contacts, difl'er 90 of the angular position of thedisk or 180 of the current wave to be rectified.

In order that the X-ray tube may be'alnals, or may be left connected, itis desirable that when the tube 1s used, the anode terminal on themachine shall always be positive and the cathode terminal alwaysnegative, in sign. The direction of current flowin the transformer 5may-be reversed 'by means of the reversing switch 14 so that if acurrent of negative polarity is delivered to the anode terminal by therectifier 7, the phase of the high tension alternating current deliveredto the rectifier 7 may be changed 180 by throwing the reversing switchand thus the anode terminal may be changed to the proper positivepolarity.

connected the same way to the termi- In order to indicate which way tothrow the reversing switch 14 to get the proper positlve and negativepolarity at the anode and cathode terminals respectively, a polarityindicator 40is mounted upon the switchboard and is connected, through arectifier 41 mounted upon the end of the motor shaft, to the supplywires 12 and 13. The polarity indicator 40 is a direct currentvolt-meter in which the indicating needle has its zero position in themiddle of the scale, theends of which are marked Up and Down respectively. The polarity indicator 40, like most direct currentvolt-meters, has a permanent magnet, and when the current flows in one.direction through the volt meter the-' needle will swing to the positionmarked Down, and when the current flows in the opposite direction theneedle will swing to the positionvmarked Up. The rectifier 41 is a diskof insulating material having four metallic contacts 42,43, 44 and 45spaced 90 apart around its periphery, the contacts 42 and 45 and thecontacts 43 and 44 being connected together in pairs as indicated inFig. 1. Disposed around the periphery of the rectifier disk and bearingagainst its periphery are four brushes 46, 47, 48 and 49. The oppositelydisposed brushes 47 and 49 are connected, through lead wires 50 and 51;to the polarity indicator 40, and the oppositely disposed brushes 46 and48 are connected, through lead wires 52 and 53 respectively, to thesupply wires 12 and 13. The rectifier 41, acts to rectify thealternating current led overthe wires 52 and 53 and delivers 'auni-directional current to the, polarity indicator over the wires 50 and51. The direction of this uni-directional current depends upon therelative position of the rotary rectifier disk with res ect to theinstantaneous polarity of the rushes 46 and 48, or, in other words, thedirection of the uni-directional current depends upon the angularposition of the motor rotor with respect to the phase of the alternatingcurrent wave. a

The connections with the polarity indicator 40 are such that when theindicating needle of the polarity indicator points to. Down, the angularposition of the motor rotor with relation to the phase of the currentwave is such that the reversing switch The polarity indicator 40 alsoserves as a synchronizer to indicate when the motor 6 is brought intostep with the current, and consequently, when to throw the startingswitch 22 from starting to running position. As the motor comes up tosynchronism, the needle of the polarity indicator vibrates, the rapidityof vibration decreasing as the motor speed approaches that ofsynchronism. The rotary rectifier 41 operates, when the motor is runningat synchronous speed, to cut off the crests of the current waves anddeliver them to the polarity indicator 40. Consequently, when the needleof the polarity indicator remains stationary at its maximumdisplacement, the operator, who is watching the needle, knows that themotor is running at synchronous speed, and he may then throw thestarting switch 22 from starting to running position.

A detailed description of some of the parts which have already beengenerally described will now be given.

The transformer 5 is a high tension transformer of the cone type. Thelaminated iron core 60 has two legs 61 and 62 connected by end yokes 63and 64. Upon the leg 62 is Wound the secondary 65 of forty-four thousandturns, which is divided into four divisions of coils of eleven thousandturns each. F orty-four turns of the secondary, located at about themiddle of the leg 62, are of heavier wire thanthe remainder of thesecondary, as indicated by the heavier line 66 in Fig. 1. The section ofheavier wire is used because it carries both the current for the X-raytube and the current for the volt meter. Taps are brought out from eachend of this section of forty-four turns of heavier wire and areconnected, through lead wires 67, with the volt-meter 20. Thevolt-meter, therefore, gets one thousandth of the total secondaryvoltage. The heavier turns 66 are located at the middle of thesecondary, where the magnetic leakage is the least, and where the bestgeneral average of the voltage induced in the seconary can be taken.Thevolt-meter 20 reads directly in volts, and by multiplying the readingby one thousand the entire secondary voltage is got. One of thevoltmeter lead wires 67 is grounded, through a safety conentiresecondary transformer.

nection 68, to the primary winding of the transformer and thusto thesupply circuit, so that the danger of high voltage discharges from thevolt-meter is guarded against; The safety connection 68 is approximately at the middle of the entire secondary winding, so thatthe'voltage between either'the anode or cathode terminals A andC andground is only half that of the A second safety connection 87 isprovided to ground the transformer core 60 upon the primary winding ofthe transformer and so prevents the accumulation of any high tensioncharges on the transformer core.

The value of the safety connection between the leadwires 67 and theprimary winding of the transformer resides in the fact that thisconnection, which is a permanent part of. the self-contained apparatus,always necessarily results in grounding the voltmeter upon the source ofcurrent by which the transformer is energized, so that it is impossibleto connect up the apparatus for use without automatically protecting thevoltmeter by this ground. Any potential which may be conducted to thevoltmeter from the source of alternating current by which thetransformer is energized, is so much lower than the high potentials orstatic charges against whichit is desired to guard thevoltmeter, thatthis ground or re'ct connection to cart The high tension secondary ofthe transformer is connected, through lead wires 69 and- 70, with the"oppositely disposed stationary contacts 71 and 72 of the rotaryconnection is practically as good as a directifier 7. The primaryWinding of the transformer is divided into two sections 81 and 82 whichare always in circuit, and four regulating sections 83, 84, 85 and 86which, by means of the voltage regulator 16, may be thrown into serieswith the sections 81 and 82. The section 81 consists of thirtyseventurns of wire wound on the same leg 62 as the secondary winding andinside of the secondary winding. The section 82 consists of forty-threeturns of wire wound on the. other leg 61. There are, therefore, eightyturns always in circuit, and these eigthy turns are about equallydivided between the le 62 upon which the secondary is wound an theopposite leg 61. The arrangement of primary turns is such that althoughwhen the transformer secondary is short-circuited by a short circuitconnection between the anode and cathode terminals, the transformer isnot dangerously overloaded or burned out, nevertheless, the magneticlinkage between theprimary'and secondary is such as to make an efficienttransformer. The primary section 81 which is wound beneath the secondaryhas very good magnetic linkage with the secondary, while the primarysection 82 on the other leg has,

by reason of the magnetic leakage, a poor magnetic linkage with thesecondary as compared with the section 81. The section 82 thus actssomewhat after the manner of resistance 3 is deg would not permit the3:1 as large secondary on rangclnent illustrated. vision of the primaryturns as herein described, the greatest secondary current compatiblewith safety against damage upon short circuiting is attained. Theregulating turns 83, 84, 85 and 86 are used only when it is desired todecrease the secondary voltage and they are, therefore, placed on theleg 61 where they have a comparatively poor magnetic linkage with thesecondary. They, therefore, not only do not occupy the more valuablespace next the secondary, but also they are thrown into circuit toreduce the voltage, and act somewhat after the manner of choke coilstoregulate the current.

One end of the primary winding is connected, through the lead wire 90,to the rotatable contact arm 91 of the adjustable rheostat or resistance3. The resistance is connected, through a lead wire 92, to the timeswitch 15 and has a number of taps 93 brought out to contacts 94 overwhich the arm 91 is moved to throw more or less resistance in serieswith the transformer to regulate the current. The sum total of the abouteighteen ohms.

From the ends of the primary transformer coils 83, 84, 85 and 86lead-wires 95, 96, 97, 98 and 99 are brought out to contacts 100, 101,102, 103 and 104 of the voltage regulator 16. The movable arm 105 of thevoltage regulator is connected, through a lead wire 106, to the timeswitch 15. The movable arm 105 is moved over the contacts to throw moreor less of the regulating sections into series with the primary and thusvary the secondary voltage of the transformer. When the contact arm 105is on the contact 100 and all of the regulating turns are cut out ofcircuit, the secondary voltage is one hundred and twenty thousand volts.As the contact arm is moved onto the contact 101, the secondary voltageis reduced to one hundred and ten thousand volts. When on the contact102, the secondary voltage is one hundred thousand volts. When oncontact 103, the secondary voltage isseventy-five thousand volts. Whenon contact 104, the secondary voltage is fifty thousand volts. Beyondcontact104 is an auxiliary contact 107 which is connected, through afive-ohm resistance 108, to the contact 104. It will be evident that theresistance 108 can be put in series with the transformer primary onlywhen all of the regulating turns are in series and the transformer isconsequently taking in less current than when the regulating coils arenot in series. Consequently, by virtue of its position, the resistance108 is never called upon to carry as great a current load as theresistance 3, and can, therefore, be made of much finer wire,thuscheapening its construction. The resistance 108 The primary circuitof the transformer i."

arranged to be broken by the time Switch 15, which is controlled by atiming device 21 which can be set to open the switch-15 ter apredetermined number of seconds exposure to the X-rays. The time switch15 is a two-pole single-throw switch, the two blades 120 and 121 ofwhich are connected, through the rheostat 3 and voltage regulator 16, tothe transformer. The two stationary switch contacts 122 and 1230f thetime switch are connected with the switch blades of the reversing switch14. The time switch has two springs 124 which tend to 0 en the switch.The time switch is held osed, against the tension of the springs 124, bymeans of a pivoted latch 125 which projects through the switchboardpanel. The latch 125 is arranged to be released by the timing device 21.The connection between the latch and the timing device 21, andv theconstruction of the timing device are not shown, as such constructionsare old and well-known in the art and constitute no part of the presentinvention. On the face of the timing device 21 is a scale divided intosec onds and a pointer or arm 127 which is movable over the scale. Thepointer 127 can be moved over the scale and set at any number ofseconds, and thereafter when the timing device is set into operation itoperates, at-

vter the number of seconds atwhich the pointer is set, to lift the latch125 and allow the time switch 15 to open. The timing de- I vice 21 isset into operation by an electromagnet 130 which is connected, throughthe lead wire 131, to the switch blade 120 and, through the lead wire132 and the singlepole single-throw switch 133, to the switch blade 121.The timing device, although set, does not begin to operate until currentpasses through the electro-magnet 130. When the tube is to be runcontinuously and the timing device is not desired, the switch 133 isopened so that the timing device is not set into operation. When a shortand definitely timed exposure is desired, the switch 133 can be closed,the pointer 127 set at the desired number of seconds, and the timeswitch 15 closed. As soon as the time switch 15 is closed, theelectro-magnet 130 sets the timing mechanism' into operation and theswitch 15 is opened after the desired time.

conducting. plates or bridging conductors 143 and 144, secured to theside of the disk 140 by means of bolts 145. These bridging conductorseach span about 90 of the periphery of the disk. While the bridgingconductors might be arranged'to span the 90 of the periphery of the diskby an arrangement of contact plates spaced 90 apart and connectedtogether by wires similar to the construction of the rectifier disk 41,it

is, nevertheless, preferred to use the two ar-' cuate conducting platesillustrated, which extend along the periphery of the disk and areconsequently at a maximum distance from the shaft throughout theirlength. The edge of the disk behind each of the plates 144 is. cut awayat 146 to prevent the arc formed upon breaking the contact from runningalong the surface of the disk. The contacts 71, 72, 141 and 142 aredisposed 90 apart about the periphery of the disk 140 and close to,but'not touching, the periphery of the disk. With the high tensionvoltage employed, the current has no difliculty in jumping from thestationary to the movable contacts. The oppositely disposed contacts 71and 72 are connected to the secondary of the transformer, and theoppositely dis posed stationary contacts 141 and 142 are connected,through the lead wires 150 and 151, to the anode and cathode terminals Aand C respectively of the X-ray tube which constitutes the load. Whenthe motor is in synchronism and the switch 14 is thrown into theposition indicated by the polarity indicator 40, the stationary contact71 is positive and the stationary contact 72 is negative at the crest ofthe wave, when the bridging conductors 143 and 144 are positioned,as'shown in Fig. 1, to connect the.

stationary contacts 71 and 72 with the stationary contacts 141 and 142respectively. During the next quarter revolution of the rectifier disk140 the current changes by onehalf a complete cycle, and the stationarycontacts 71 and 72 are negative andpositive respectively, when thebridging contacts 143 and 144 connect them with the stationary contacts141 and 142 respectively. The rectifier, therefore, acts to take thehigher voltage part. of each wave of the high tension alternatingsecondary current of the transformer and to deliver the crest of all ofthe positive waves at the anode terminal through the contact 141 andconductor 150, and to deliver the crests of all the negative waves atthe cathode terminal through the contact 142 and conductor 151. As aboveexplained, the polarity indicator40 tells the operator which way tothrow the reversing switch 14'to have a positive polarity at the anodeterminal A, and continues to indicate the polarity of the anode andcathode terminals. If the reversing switch should be thrown Up when thepolarity indicator points Down, the anode would have a negativepolarity, and the needle, by continuing to point Down while thereversing switch is Up, would continue to notify the operator that theanode and cathode terminals had'the wrong polarity.

The stationary contacts 71, 72,141 and 142 are mounted upon the endsoffour hard rubber posts 152 Which roject from four brackets 153 on theend of the motor frame. Three of the brackets 153 are of metal, but thefourth bracket which, as indicated in Fig. 4, projects toward the panel4, is of' hard rubber to guard against leakage dis-' charges between itand the high tension terminals. The lead wires which go from cgo thestationary rectifier terminals to the trans-former and to the anode andcathode terminals are inclosed in hard rubber tubes 154. y In order torigidly space the stationare similar-in construction and project fromthe face of the hard rubber panel 4. The construction of these terminalsis shown in more detail in Fig. 5, which is a section of the anodeterminal. A hard rubber bushing 160 fits through a hole in the hardrubber panel 4 and is clamped in place by two collars 161 and 162threaded on the bushing 160 and bearing against the front and back ofthe panel. In the end of the bushing 160 is u a socket in which isreceived the end of a hard rubber sleeve 163. The sleeve 163 makes africtional fit in the bushing socket and is held from rotation thereinby means of a spline 164. Extending throughv the bushing 160 is aconducting sleeve 165 having a flanged forward end to keep it from beingpulled backwardthrough the bushing. The sleeve 165 is firmly clamped inposition by a nut 167 threaded on the sleeve. Clamped beneath the nut isa terminal piece 168 to which the high tension lead 150 is.

soldered. Rotatably mounted in the sleeve 165 is a conducting rod 170.The rod 170 carries, at its rear end, a hard rubber pulley 171. Theforward end of the rod 170 is enlarged to form a socket 172 in which isfrictionally held the split end of a second rod 173 which is looselymounted to rotate in the hard rubber sleeve 163. The end of the sleeve163 is internally threaded to receive a thimble 174 intowhich the end ofthe conducting rod 173 loosely fits.v The outer end of the thimble 174has a hole through I which a flexible conducting cord 175 can beattached. A collar 17 6 is mounted to rotate on the thimble 174.Threaded in the radially gap arm extends into ble rod 17 3.- The upperpart of the thimble 17 1 is cut awayat 1 9 so that the arm 177 is treeto turn rather more than half a circle.

The cut away portion 17 9 is formed by saws ing partially t rough thethimble 17d, leaving the bottom ortion 180 of the thimble below the cut.ihe conducting cord 17 5 extends from the anode terminal to a hot wiremilliammeter 181 mounted on a bracket 182 upon the casing. On the sideof the milliammeter is a spring-held spool 183 from which can be drawnout a flexible metallic tape 184k for connection with the anode terminalof the X-ray tube 10. The construction of the cathode terminal issubstantially the same as that of the anode terminal, the onlydifference bein that the spool 18?) which carries the flexi le tape 184:for the cathode terminal of the X-ray tube is mounted upon the spark arm185 of the cathode terminal. 0n the rear of the cathode terminal is ahard rubber pulley 186 similar to the pulley 171 of the anode terminal.The pulleys 171 and 186 are connected together and to the pulley 188 bymeans of string belts 187. The pulley 188 is turned, through beveledgears 189, by means of a hard rubber handle 190 at the front of themachine. When the handle 190 is turned, the pulleys 171 and 186 turn thearms 177 and 185 toward or from each other to vary the spark gap betweentheir ends, so that the sparking distance of the current can bedetermined.- On the lower part of the hard rubber panel is mounted aregulating arm R for use with X-ray tubes which have a chemical chamberfor lowering the vacuum. The construction and mounting of the regulatingarm R' is shown in detail in Fig. 6 and resembles that of the anode andcathode terminals. Like the anode terminal, the regulating arm R ismounted in a collar 200 which is rotatably mounted on a thimble 201threaded on the end of a hard rubber sleeve 202. The arm R is turned bymeans of rods 203 and 20 which extend through the sleeve 202 and bushing205. A pulley 206 is mounted on the rear of the rod 204 and isconnected, by a string belt 207, to a pulley 208- which is turned,through beveled gears 209, by a hard rubber handle 210. By means of thehandle 210., the arm R can be swung up toward the cathode terminal C tolet a shunt current flow through the chemical chamber 220 of the X-raytube in order to lower its vacuum.

. In order that an excessive current may not pass through the chemicalchamber 220, a high resistance 230 is inserted between the chemicalchamber 220 and the regulating knob 225 and brings the regulating arm Rarm B. This high resistance consists of a mixture 221 of graphite andlaster of Paris inclosed in a glass tube 222 which, in turn, is inclosedin a hard rubber tube 223 which is threaded on the end of the thimble201. 'A spring-pressed plunger 224 acts to hold the powdered materialtogether. The springpressed plunger 224 is mounted in, and iselectrically connected with, a terminal knob 225. The terminal knob 225has a hole through which is passed a flexible conductor 226 which leads,to the chemical chamber 220 of the X-ray tube. In order to lower thevacuum of the tube, the operator turns the toward the cathode terminal Cuntil the current jumps the gap between the end of the arm R and thecathode terminal. The passage of the current liberates the gas from thematerial, preferabl asbestos wool, and 8 after the vacuum is su cientlyreduced, the o erator swings the arm R out of sparking dlstanre from thecathode terminal.-

While the preferred embodiment of the present invention has beenspecifically illustrated and described, it is to be understood that thepresent invention is not limited to such embodiment, but may be embodiedin other constructions within the scope of the invention as set 1 forthin the following claims 1. The combination of a self-contained X- rayapparatus comprising a step-up transformer, and terminals by which thetransformer may be connected with. an external source of current; anelectric instrument; and conductors for connecting the instrument withthe secondary winding of the transformer to indicate an electricalcondition therein; one of said conductors bein connected, by conductorsembodied in sai self-contained apparatus, with. one of said terminals,so as to ground the instrument automatically upon the source of currentwhenever the latter is connected with the 1 "said terminal.

2. Self contained X ray apparatus having, in combination, a step-uptransformer; terminals by which the transformer may be connected with anexternal source of cur- 1 rent; conductors by which an electricinstrument may be connected across a few turns of the secondary windingof the transformer, at a point of substantially zero potential; andapermanent connection between 1 one of said conductors and the primary-windingof the transformer, whereby an instrument connected as aforesaidmay be grounded upon the source of current by Way of the primary windingand the terminals. 1 3. A high potential rectifying commutatorcomprising a rotary member of insulating material having oppositelydisposed arcuate conducting plates each extending along about 90 of theperiphery of the 1 member, and four stationary contacts located I about90 .apart around the member.

4. A high potential rectifier comprising a. rotary member of insulatingmaterial having oppositely disposed bridging conductors at itsperiphery, a synchronous motor for driving the rotary member havingashaft on which the rotary member ismounted, stationary contacts mountedaround the periphery of the member, and supportin means for thestationary contacts eXten ing directly and rigidly from the motorcasing.

5. A high potential rectifying commutator having, in combination, arotary member of insulating material having oppositely disposed bridgingconductors spanning about 90 of the periphery of the member, theperiphery of said insulating member being cut away behind each bridgingconductor, a motor for driving the member and four stationary contactslocated about 90 apart around the member.

6. A high potential rectifier comprising a rotary disk of insulatingmaterial having oppositely disposed bridging conductors spanning about90 of the periphery of the disk, a motor for driving the disk having ashaft on the end of which the disk is mounted, four stationary contactslocated about 90 apart around the disk, and means for rigidly supportingthe stationary contacts including four posts of insulating materialextending from the motor casing and rods of insulating materialconnecting adjacent posts for bracing them.

7. A high potential rectifier comprising a rotary disk of insulatingmaterial having oppositely disposed bridging conductors spanning about90 of the periphery of the disk, a synchronous motor for driving thedisk having ashaft on the end of which the disk is mounted, fourstationary contacts located about 90 apart around the disk, and meansfor rigidly supporting the stationary contacts including four posts ofinsulating material extending from the motor casing.

CHARLES E. CAMPBELL. Witnesses I s H. FESSENDEN, GEO. E. STEBBINS.

